Apparatus for finishing metallic coatings

ABSTRACT

Apparatus for finishing a molten metallic coating on a cylindrical strand wherein a swirling gas jet is directed at a moving strand carrying still molten coating metal so as to remove excess coating metal and break up pneumatically the oxide skin on the coating metal into small, uniformly distributed particles, including a nozzle having a plurality of slots equally spaced about the strand pass line and directed tangentially toward a strand passing therethrough.

United States Patent Inventor Marvin B. Pierson Middletown, Ohio Appl. No. 22,423

Filed Mar. 25, 1970 Division of Ser. No. 708,598, Feb. 27, 1968, Pat. No. 3,533,761

Oct. 12, 1971 Armco Steel Corporation Middletown, Ohio Patented Assignee APPARATUS FOR FINISHING METALLIC COATINGS 5 Claims, 3 Drawing Figs.

US. Cl 118/63, 34/160 Int. Cl B05c 11/06 Field of Search 1 18/D1G. 18,63,325; 34/155, 156, 160; 239/463,468;

Primary Examiner-Morris Kaplan Att0rneyMelvillc, Strasser, Foster & Hoffman ABSTRACT: Apparatus for finishing a molten metallic coating on a cylindrical strand wherein a swirling gas jet is directed at a moving strand carrying still molten coating metal so as to remove excess coating metal and break up pneumatically the oxide skin on the coating metal into small, uniformly distributed particles, including a nozzle having a plurality of slots equally spaced about the strand pass line and directed tangentially toward a strand passing therethrough.

APPARATUS FOR FINISHING METALLIC COATINGS CROSS-REFERENCE TO DIVISIONAL APPLICATION This is a division of application for U.S. Pat. Ser. No. 708,598, filed Feb. 27, 1968, entitled Method and Apparatus for Finishing Metallic Coatings, now US Pat. No. 3,533,761.

BACKGROUND OF THE INVENTION This invention relates generally to the coating of a cylindrical strand such as wire or tubing with any of the conventional metallic coatings including aluminum and its alloys, zinc and its alloys teme, and the like.

The continuous hot dip coating methods according to the prior art have endeavored to produce an oxide free coating. That is, these prior art coating methods have utilized a variety of finishing means such as dies, rolls, wipers, gas jets, and the like which substantially prevent coating metal oxides formed prior to the finishing operation from passing the finishing means and exiting on the product. It should be understood that the molten metal surface will oxidize immediately after the product leaves the finishing means, however, as used in this application, the term oxide free will refer to a coating on which no oxide skin is visible from a normal visual examination.

More specifically, it is recognized in the art that the exposed surface of the bath of molten coating metal develops an oxide or scum layer. This is particularly true in the case of aluminum, wherein the oxide skin is unusually tough and gummy. The efforts of workers in the prior art have been to devise a method wherein the strand could be withdrawn from the bath of molten metal without picking up this oxide layer.

For example, U.S. Pat. Nos. 2,9l4,423 and 3,060,889, both in the name of Earle L. Knapp, contemplate a process wherein the wire is withdrawn vertically from the coating metal bath, so that the moving wire will pull up the molten coating metal to a portion of the finishing die. An oxide sock, will extend from the bath to the die, while the substantially pure metal oxide free) under the oxide layer is withdrawn as a coating on the wire.

A horizontal coating operation utilizing a coating metal container having close tolerance entry seal and exit dies permits passage of the strand through the bath without contacting the oxide layer on the surface of the bath. By restricting the area of the exposed coating metal meniscus, and by directing a nonoxidizing gas with sufficient mass and velocity against the meniscus, an oxide free coating can be successfully produced. However, this technique has not been successfully used to produce heavier coating weights (coating weights in excess of 0.30 ounces per square foot of surface for zinc).

However, under certain circumstances such as in the coating of copper plated tubing, it is possible to use close tolerance dies for the coating bath, thereby necessitating a flooding technique as described in copending application U.S. Pat. No. 616,680, in the name of Kenneth G. Coburn et al. While this flooding technique eliminates the problem of close tolerance dies, it also means that the molten coating metal is exposed to the atmosphere before finishing is accomplished, and hence the problem of oxide control becomes a factor once again.

As already indicated in connection with vertical coating methods, the prior art has heretofore thought that the only solution would be found in devising a method for removing, eliminating, or restraining this oxide layer.

The instant invention, in its broadest aspect, depends upon exactly the opposite concept. That is, this invention is directed to the production of what might be called an oxide finish" coating. An oxide finish" coating is produced when the relatively heavy, visible oxide formed on the molten coating metal prior to and during finishing is applied to the strand by the finishing means. As previously indicated, this type of coating has heretofore been largely undesirable.

Keeping the foregoing comments in mine, it is a primary object of this invention to provide an apparatus for finishing a metallic coating so as to produce a heavier coating of uniform coating thickness, both longitudinally and transversely of the strand than previously possible.

It is a further object of this invention to provide an apparatus for producing a smooth, oxide finish metallic coating on a cylindrical strand.

SUMMARY OF THE INVENTION In its broadest concept, this invention contemplates that a moving strand is passed through a bath of molten coating metal. Some distance from the coating bath, a swirling gaseous jet is positioned which, by virtue of its twisting action, causes the surface oxide to be broken into very small uniformly distributed particles.

The swirling gas jet also is utilized to create a dam or air knife effect to control the quantity of molten coating metal adhering to the strand, and hence controlling coating weight.

The apparatus of this invention includes a jet nozzle having a plurality of slots radially spaced about the opening, and directed tangentially toward the strand passing through the nozzle.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing a portion of a metallic coating line utilizing the apparatus of this invention.

FIG. 2 is a cross-sectional view showing the nozzle of this invention.

FIG. 3 is an end elevational view of one of the components of FIG. 2 showing the tangential slot pattern.

DESCRIPTION OF THE PREFERRED EMBODIMENT In general, the hot dip coating of wire, tubing, and the like includes the steps of thoroughly cleaning or otherwise preparing the surface of the base metal for the reception of the molten coating. These preparatory steps do not per se form a part of this invention, and it is to be understood that this invention contemplates such a pretreatment prior to the time the wire arrives at the molten coating bath.

Exemplary preparatory procedures now in commercial use are described in detail in Sendzimir U.S. Pat. Nos. 2,! 10,893, 2,136,957, and 2,197,662, and in the copending application 616,680 referred to earlier. In general, these references contemplate that the strand is cleaned of oils, greases and the like by passage through an oxidizing furnace wherein carbonaceous material is burned from the surface, and a very thin controlled oxide coating is formed thereon. The strand is then subjected to a heat treatment in a reducing atmosphere wherein the previously formed oxide layer is removed. Finally, without reexposing the wire to atmosphere, it is passed directly into a bath of molten coating metal.

Instead of the described treatment in an oxidizing furnace, alkali or other chemical cleaning involving wetting and drying of the strip surface, or even abrasive treatment may be used, so long as the surface of the wire is sufficiently cleaned that an extremely rapid and thorough wetting by the molten metal itself takes place in the coating bath.

Turning now to FIG. 1, the strand being coated is indicated at 10. As previously indicated, it is to be assumed that the surface of the strand has been thoroughly cleaned by an appropriate precoating treatment. The tube 12 extends between the reducing furnace and the coating bath, so as to provide a suitable protective atmosphere for the strand prior to immersion in the coating bath.

The coating bath is schematically indicated at 14. As is well known in the art, this coating bath will be provided with entrance and exit dies aligned so as to define a path of travel for the strand being coated. (While the illustrated examples shows a horizontal path of travel, the invention is equally applicable to a vertical coating operation.) According to this invention, the entrance die should provide relatively close clearance for the strand being coated in order to prevent excessive leakage of the coating metal.

By comparison, the exit die indicated at 16 in H0. 11 should provide considerable clearance for the passing strand in order to insure that an excess of coating metal is carried out by the moving strand. This excess of coating material carried by the strand is schematically shown at 18.

The excess of coating metal may be applied in various ways besides exiting from a loose tolerance die. For example, the molten coating metal may be poured onto the passing strand as taught in copending application No. 616,680 noted earlier, or it may be flooded onto the strand by passing the strand through a trough containing molten coating metal.

The apparatus of this invention comprises a jet nozzle indicated generally at 20 located a substantial distance away from the exit die 16.

The jet nozzle is shown in more detail in FIGS. 2 and 3. it comprises a female element 22 having a cylindrical bore 24 terminating in the conical wall 26. As seen in FIG. 2, the righthand portion of the bore 24 is threaded as at 24a. The element 22 is also provided with an aperture 28 through which the strand being coated passes.

The male element of the jet nozzle is indicated generally at 30, and includes the threaded portion 32 which will mate with the threads 24a of the female element. The element is also provided with a portion 34 of reduced diameter, which terminates in the conical surface 36. lt will be apparent that the angle of the conical surface 36 on the end of the male element 30 is identical to the angle of the conical surface 26 at the end of the bore in the female element 22. It will also be seen that the male element is provided with a central aperture 38 of the same size as the aperture 28 in the female element, and through which the strand being coated passes.

Referring now to FIG. 3, it will be observed that the conical surface 36 of the male element is provided with a plurality of slots 40. These slots are arranged so that their axes are substantially tangent to the external surface of the tube being coated.

ln operation of this nozzle, the male element 30 is threadedly received into the female element 22. This arrangement permits the provision of an adjustable annular slot opening 42. The area between bore 24 of the female element and the portion 34 of reduced diameter of the male element, in effect provides a plenum chamber which will be supplied with a suitable wiping gas through the tube 44. By virtue of the angular positioning of the slots 40, the nozzle of this invention provides a spiraling or twisting jet action upon the excess coating metal carried by the strand 10.

While applicant does not intend to be bound by theory, it is believed that this spiraling, twisting jet action on the molten coating metal is twofold. First of all, it is believed that the jet creates a dam effect or air knife action which will prevent ex cess molten coating metal from passing through the nozzle. This action is similar in effect to a conventional jet or air knife wiper.

The second feature or effect of this twisting jet is believed to represent a substantial departure from the practice of the prior art. That is, as indicated at the outset of this specification, the excess molten metal on the strand rapidly develops an oxide skin, referred to in the art as an oxide sock. With a conventional concentric jet nozzle, using high enough pressures and placed close enough to the exit die of the coating bath, it is possible to restrain this oxide sock along with most of the excess molten coating metal applied to the strand, resulting in a relatively light, oxide free metallic coating. As efforts are made to produce a heavier coating, the conventional jet permits an intermittent breaking of the oxide sock, causing an unsightly finish and nonuniform coating. Usually, when an oxide ringlet or segment breaks off, the coating thickness immediately following the ringlet is significantly less than normal. There is also a periodic tendency for the whole oxide sock between the coating bath and the nozzle to build up, break off, and pass totally through the nozzle on the tube. It is believed that the swirling gas jet of this invention eliminates these shortcomings by providing a turbulent twisting action which causes the oxide sock to break up continuously into very small segments or particles which remain on the coating. Thus, the instant invention represents the first successful application of an "oxide finish coating. The' oxide formed ahead of the finishing means is intentionally and continuously applied to the product in a controlled manner.

It has also been found that the apparatus of this invention is capable of producing far heavier coatings on a horizontal pass without appreciable sag of coating metal with respect to the strand. This excellent transverse unifonnity of coating is believed to be achieved in part by the redistribution of coating metal caused by the swirling, twisting action of the jet, in part by the chilling action of the jet, and in part the oxide skin finish which tends to support the molten metal film until solidification is completed in a subsequent water quench.

ln exemplary operation utilizing the nozzle of this invention in connection with the coating of a 55-inch outsidexiiameter tubing, the preferred design for the nozzle is as follows: the diameter of the apertures 28 and 38 should be five-eighths inches; the angle of the conical surfaces 26 and 36 to the tube 10 being coated should be eight slots 40 were provided in the conical surface 36, each of the slots being l/-l6-inch wide and 1/1 6-inch deep, and tangent to a 3/8-inch diameter circle centered in the bore. (The direction of the swirling action is unimportant.)

As indicated earlier, the threaded engagement between the female and male elements 22 and 30 respectively provides an adjustable annular slot opening 42. It has been found that this annular opening is useful to minimize splatter of the coating metal at low speeds. However, at speeds on the order of 50 feet per minute and faster, better operation is obtained when the annular opening is sealed off by threading the male element 30 into the female element until it is fully seated.

Preheated air, superheated steam, or other oxidizing gases are preferred as the wiping agent since the oxide produced on the coating is beneficial in resisting the sagging of the coating. Other gases such as nitrogen and argon can be used.

The plenum pressure can be varied between k and 3 psi, and is the basic control for coating weight on the unit. It has been found that this apparatus, operating horizontally and using the pressure range set up above can successfully produce coating weights from about 0.10 to about 0.60 ounces per square foot of strand surface without observable coating metal sag.

Returning now briefly to FIG. 1, the action of the jet nozzle on the excess coating has been schematically illustrated. The excess coating metal will run off the strand into a reservoir 46. This material is then conveyed by means of the pump 48 back to the coating bath 14.

It is believed that the foregoing description constitutes a full explanation of the invention. While the invention has been described in terms of an exemplary embodiment, no limitations are to be inferred or implied except as specifically set forth in the claims which follow.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A nozzle for finishing a molten metallic coating on a strand comprising:

a. a female element having a bore terminating in a wall;

b. a male element adapted to be received in said bore in said female element and having a portion of reduced diameter, defining in cooperation with said bore, a plenum chamber;

c. means communicating said plenum chamber with a supply of fluid under pressure;

d. a passageway extending axially through said male element and said wall of the female element and adapted to accommodate a freshly coated strand passing therethrough;

e. the inserted end-face of said male element cooperating with said end wall to define a narrow annulus communicating with said passageway and with said plenum chamber and adapted to direct a generally annular stream of said fluid onto the strand passing therethrough; and

3. The apparatus claimed in claim 1, wherein said wall of said female element is conical.

4. The apparatus claimed in claim 1, wherein said portion of said male element of reduced diameter is adjacent said end wall.

5. The apparatus claimed in claim 1 wherein said means defining a plurality of passages comprise a plurality of slots disposed in the end-face of the male element. 

2. The apparatus claimed in claim 1, wherein said end wall of said male element is conical, said passage means in said male element being disposed in said conical end.
 3. The apparatus claimed in claim 1, wherein said wall of said female element is conical.
 4. The apparatus claimed in claim 1, wherein said portion of said male element of reduced diameter is adjacent said end wall.
 5. The apparatus claimed in claim 1 wherein said means defining a plurality of passages comprise a plurality of slots disposed in the end-face of the male element. 